A successor communication system to W-CDMA and HSDPA, i.e., Long Term Evolution (LTE), is currently being discussed by 3GPP, a standardization group for W-CDMA. In LTE, orthogonal frequency division multiplexing (OFDM) is to be used as a downlink radio access method and single-carrier frequency division multiple access (SC-FDMA) is to be used as an uplink radio access method (see, for example, 3GPP TR 25.814 (V7.0.0), “Physical Layer Aspects for Evolved UTRA,” June 2006).
In OFDM, a frequency band is divided into multiple narrow frequency bands (subcarriers) and data are transmitted on the subcarriers. The subcarriers are densely arranged along the frequency axis such that they partly overlap each other but do not interfere with each other. This approach enables high-speed transmission and improves frequency efficiency.
In SC-FDMA, a frequency band is divided into multiple frequency bands and the frequency bands are allocated to different terminals for transmission in order to reduce interference between the terminals. Also, SC-FDMA reduces variation of the transmission power and therefore makes it possible to reduce power consumption of terminals and to achieve wide coverage.
A reference signal for uplink in E-UTRA indicates a pilot channel that is used for purposes such as synchronization, channel estimation for coherent detection, and measurement of received SINR in transmission power control. The reference signal is a transmission signal known to the receiving end, i.e., the base station, and is embedded at intervals in each subframe.
In W-CDMA, a user-specific PN sequence, more precisely, a signal sequence obtained by multiplying a long-cycle Gold sequence and an orthogonal sequence, is used as the reference signal (pilot channel). Since the PN sequence is long, it is possible to generate many different PN sequences. However, since the correlation properties of PN sequences are poor, the accuracy of channel estimation may become low. In other words, the interference between a pilot channel of a user and a pilot channel of another user may become high. Also, in a multipath environment, the autocorrelation between a pilot channel sequence and its delayed wave becomes high. In W-CDMA, simple reception processing such as RAKE reception is employed. Meanwhile, an E-UTRA system is designed to suppress the multipath interference based on highly-accurate channel estimation using, for example, an equalizer. For this reason, in E-UTRA, a constant amplitude and zero auto-correlation (CAZAC) sequence is used instead of a user-specific PN sequence.
The CAZAC sequence has excellent autocorrelation properties and cross-correlation properties and therefore enables highly-accurate channel estimation. In other words, compared with the PN sequence, the CAZAC sequence makes it possible to greatly improve the demodulation accuracy. With the CAZAC sequence, the variation in the amplitude of a signal is small both in the frequency domain and the time domain, i.e., the amplitude of the signal becomes comparatively flat. Meanwhile, with the PN sequence, the variation in the amplitude of a signal is large in the frequency domain. Thus, using the CAZAC sequence makes it possible to accurately perform channel estimation for each frequency using an equalizer. Also, since the autocorrelation of a transmitted CAZAC sequence becomes zero, it is possible to reduce the influence of multipath interference.
Still, the CAZAC sequence has problems as described below.                The number of sequences is small.        
Since it is not possible to assign unique CAZAC sequences to respective users, it is necessary to repeatedly or cyclically assign a limited number of CAZAC sequences to multiple cells (hereafter, this is called “cell reuse”). The number of sequences becomes particularly small when the transmission band in SC-FDMA is narrow. In other words, when the transmission band in SC-FDMA is narrow, the symbol rate becomes low and the CAZAC sequence length decreases. In E-UTRA, a reference signal is time-division-multiplexed. Therefore, the symbol rate becomes low and the sequence length decreases when the transmission band is narrow. The number of sequences corresponds to the sequence length. For example, when the sequence length is 12 symbols in a transmission band of 180 kHz, it is not possible to assign user-specific sequences and therefore it is necessary to repeatedly or cyclically assign 12 sequences to multiple cells (may be greater than 12) such that the same sequence is not assigned to neighboring cells.                Cross-correlation between CAZAC sequences with different lengths varies rather greatly depending on the combination of the CAZAC sequences. When the cross-correlation is high, the accuracy of channel estimation is reduced.        
Next, SC-FDMA used as an uplink radio access method in E-UTRA is described with reference to FIG. 1. In SC-FDMA, a system frequency band is divided into multiple resource blocks each of which includes one or more subcarriers. Each user device (user equipment: UE) is allocated one or more resource blocks. In frequency scheduling, to improve the transmission efficiency or the throughput of the entire system, resource blocks are allocated preferentially to user devices with good channel conditions according to received signal quality or channel quality indicators (CQIs) measured and reported based on downlink pilot channels for the respective resource blocks by the user devices. Frequency hopping where allocation of frequency blocks is varied according to a frequency hopping pattern may also be employed.
In FIG. 1, time and frequency resources allocated to different users are represented by different hatchings. For example, a relatively wide frequency band is allocated to UE2 in the first subframe, but a relatively narrow frequency band is allocated to UE2 in the next subframe. Different frequency bands are allocated to the respective users such that the frequency bands do not overlap.
In SC-FDMA, different time and frequency resources are allocated to respective users in a cell for transmission to achieve orthogonality between the users in the cell. Here, the minimum unit of the time and frequency resources is called a resource unit (RU). In SC-FDMA, a consecutive frequency band is allocated to each user to achieve single-carrier transmission with a low peak-to-average power ratio (PAPR). Allocation of the time and frequency resources in SC-FDMA is determined by a scheduler of the base station based on propagation conditions of respective users and the quality of service (QoS) of data to be transmitted. The QoS includes a data rate, a desired error rate, and a delay. Thus, in SC-FDMA, the system throughput is improved by allocating time and frequency resources providing good propagation conditions to respective users.
Respective base stations separately determine allocation of time and frequency resources. Therefore, a frequency band allocated in a cell may overlap a frequency band allocated in a neighboring cell. If frequency bands allocated in neighboring cells partly overlap, signals interfere with each other and their quality is reduced.
Next, a reference signal in uplink SC-FDMA is described with reference to FIG. 2. FIG. 2 shows an example of a subframe structure.
The packet length of a TTI called a subframe is 1 ms. One subframe includes 14 blocks to be submitted to FFT. Two of the 14 blocks are used for transmission of a reference signal and the remaining 12 blocks are used for transmission of data.
The reference signal is time-division-multiplexed with a data channel. The transmission bandwidth is dynamically changed according to the results of frequency scheduling by the base station. When the transmission bandwidth decreases, the symbol rate decreases and the sequence length of a reference signal to be transmitted in a fixed time period decreases. When the transmission bandwidth increases, the symbol rate increases and the sequence length of a reference signal to be transmitted in a fixed time period increases. When a reference signal is to be transmitted using a narrow band, for example, a 180 kHz band that equals one resource unit or 12 subcarriers, the number of symbols becomes 12. In this case, both the sequence length and the number of sequences become about 12. When a reference signal is to be transmitted using a wide band, for example, a 4.5 MHz band that equals 25 resource units or 300 subcarriers, the number of symbols becomes 300. In this case, both the sequence length and the number of sequences become about 300.
Meanwhile, orthogonalization of multiple reference signals by using cyclically-shifted CAZAC sequences is proposed. As shown in FIG. 3, when cyclically-shifted CAZAC sequences are used and all multipaths are within the amount of cyclic shift (cyclic shift amount), it is possible to orthogonalize reference signals of different users and antennas. Even when different users transmit the same sequence at the same timing using the same frequency band, it is possible to orthogonalize the users by cyclically shifting the sequence.
It is also proposed to orthogonalize two reference signals by employing orthogonal covering. In orthogonal covering, as shown in FIG. 4, users 1 and 2 may use different CAZAC sequences and different cyclic shift amounts as long as the same CAZAC sequence and the same cyclic shift amount are used for two reference signals in a subframe. With this approach, after the two reference signals are despread, the users become orthogonal with each other.